Hybrid binary/thermometer code for controlled-voltage integrated circuit output drivers

ABSTRACT

A hybrid binary/thermometer code is employed to adjust the output impedance of a variable impedance output driver circuit having an impedance network comprising a plurality of impedance legs each programmably electrically connectable according to the hybrid binary/thermometer code in parallel between a voltage source and the signal pad. The plurality of impedance legs are partitioned into one or more set pairs of binary stepped impedance legs and corresponding thermometer stepped impedance legs. A binary set of calibration signals in the hybrid binary/thermometer code steps a given set of binary stepped impedance legs according to a binary code and a thermometer set of calibration signals in the hybrid binary/thermometer code steps the corresponding set of thermometer stepped impedance legs according to a thermometer code once per full count iteration of the binary set of calibration signals.

BACKGROUND OF THE INVENTION

The present invention pertains generally to variable impedance outputdrivers in integrated circuits, and more particularly to a novel hybridcode for controlling the output impedance of integrated circuit outputdrivers due to variations in manufacturing process, voltage, andtemperature.

Integrated circuits are commonly packaged as chips. An integratedcircuit communicates with devices outside the chip via input and outputsignal pads on the exterior of the chip. Inside the chip, the signalpads are connected to signal receiver and signal driver circuitry, asappropriate, to receive incoming signals or to drive outgoing signals.

The signal pads on a chip are connected to the packaging of the chip(e.g., a pin) which is then typically connected to respective signaltraces on a printed circuit board. The signal traces may connect thechip to other integrated circuit chips, electronic devices, orconnectors on the printed circuit board that connect to external (i.e.,off-board) devices. The electrical connection between the signal pad ofthe integrated circuit die and signal trace of the printed circuit boardthrough the packaging of the chip is characterized by parasiticresistance, inductance, and capacitance, which interferes with thetransmission of the signal from the signal pad. The transmission linecharacteristics of the printed circuit board signal trace itself,including parasitic resistance, capacitance, and inductance, alsointerfere with the quality of the transmission of the signal from thesignal pad. All of the foregoing add to the load impedance which must bedriven by the output driver circuit.

Due to the parasitic resistance, capacitance, and inductance which ispresent on chip-to-chip signal transmission lines, the driver circuitsthat drive those transmission lines typically includes circuitrydesigned to avoid excessive voltage swings, or “ringing”, when signalswitching occurs. Ringing must be avoided while still switching as fastas possible to meet the high speed performance requirements of modernintegrated circuits.

As known by those skilled in the art, it is important to match theoutput impedance of a given signal driver to the characteristicimpedance of the transmission line it drives in order to avoid signalreflections due to voltage level switching on the pad, and thereforeundesirable signal degradation.

Matching the impedance of an output driver to the characteristicimpedance of the signal transmission line is problematic for severalreasons. First, process variations inherent in the manufacturing processof integrated circuits, such as the transistor implanting doping level,the effective length of channels in the field effect transistors (FETs),the thickness of the gate oxide for transistors, and the diffusionresistance, can cause the output impedance of two supposedly identicalcircuits to differ. In particular, variations in any or all of the aboveprocess parameters can cause different integrated circuits intended toperform the same function to be classified as “slow”, “nominal”, or“fast”. In other words, two supposedly identical integrated circuits canvary in any or all of the process parameters. As these parametersapproach the fast case, the resistance of many components within a chipis decreased. In the opposite extreme, as the process parameters strayfurther and further from the ideal case, the performance of the chip isdegraded, specifically, the resistance of the many components within thechip is increased. This situation is referred to as the “slow” case.

In addition, variations in voltage and temperature can cause variationsin the output impedance of a given chip. Specifically, the driver outputimpedance can vary significantly between variations in the operatingvoltage even within a small operating voltage tolerance range. Inanother example, when the temperature of an integrated circuitapproaches its maximum operating temperature, the resistance of theintegrated circuit components increases.

In view of the above, variable impedance output drivers have beendeveloped to allow adjustment of the driver output impedance due tovariations in manufacturing process, voltage, and temperature. Typicallythe characteristic impedance of transmission lines connected to I/O padsof the output drivers are known and/or can be measured, and once knownor measured, a precision external resistor R_(EXT) of correspondingvalue may be provided on the printed circuit board to serve as areference impedance from which to match. Alternatively, instead ofprecision external resistor R_(EXT), other types of arrangements can beused.

It is becoming standard to use a programmable impedance network to matchthe impedance of an output signal pad driven by an output driver to theexternal resistor R_(EXT). In an impedance network, a plurality ofimpedance legs, each of which is characterized by a predeterminedimpedance, are programmably connectable between a voltage source and anode coupled to an output pad driven by an output driver. As known inthe art, the term “node” refers to a pad, a trace, a wire, an electricalconductor, or any electrically connected combination and/or equivalentthereof, such that any point of the “node” is characterized by the sameelectrical state as all other points of the “node”, subject to a marginof error determined by the characteristic resistance, capacitance, andinductance between points of the node. When an appropriate combinationof parallel legs of the impedance network are actively connected betweenthe voltage source and node, such that a voltage V_(DIV) measured at areference point is one half (½) a voltage V applied across the seriescombination of the external resistor R_(EXT) and the active parallellegs of the impedance network, then it is known that the externalresistor R_(EXT) and active parallel legs of transistors within theimpedance network are equally sharing/dividing the voltage V. That is,if two series portions are equally sharing/dividing a voltage, then suchtwo series portions have the same impedance. Accordingly, as a result ofthe above method, the output impedance of the pad coupled to the outputdriver is “adapted” to external resistor R_(EXT), as the active parallellegs of the impedance network are providing an impedance (e.g.,resistance) which matches that of external resistor R_(EXT).

The above “adaptive” procedure may be performed not only uponinitialization (e.g., reset) of the integrated circuit, but may also becontinuously or periodically performed during operation of theintegrated circuit. Such continuous/periodic operation is advantageousbecause environmental parameters (e.g., voltage, temperature, etc.) ofthe integrated circuit change over time (e.g., the integrated circuitbecomes hotter with operation which changes on-die impedances), and thusthe arrangements of the present invention can be adaptive to changecontinuously/periodically.

One prior art technique for accomplishing impedance matching of outputpads for integrated circuits is described in U.S. Pat. No. 6,118,310 toEsch, Jr. and assigned to the same assignee of interest, entitled“Digitally Controlled Output Driver and Method for Impedance Matching”,herein incorporated by reference for all that it teaches. In thetechnique described therein, output driver impedance matching isaccomplished by programmably enabling a combination of FETs arranged inparallel whose combined impedance closely matches the characteristicimpedance of the transmission line.

Such prior art variable impedance output drivers typically use a purethermometer code for the PVT impedance matching control in order tolimit the change in output impedance when the PVT control code isupdated. In particular, an impedance network having n parallel legs mayimplement an n-bit “thermometer” code T_(0::n-1). The state of each bitin the n-bit code T_(0::n-1) controls activation of respective legs inthe impedance network. In a thermometer code, when a bit T_(i) of thecode T_(0::n-1) is activated (set to “1”), all of the lower-order bitsT₁ to T_(i-1) are also activated. Thus, in a pure thermometer codeimpedance matching circuit, a first impedance leg is activated and theneach subsequent impedance leg is activated until the desired outputimpedance is achieved. Accordingly, at least one impedance leg is alwaysactivated to ensure that during the switching of impedance legs on oroff, the impedance legs are never switched from all off to all on orvice versa, which would result in a spike in the output impedance. Table1 illustrates a pure 11-bit thermometer code, wherein each bit 0::10 inthe code word T represents an incremental admittance step of 10%—that iseach impedance leg is weighted by a 10% incremental impedance amount.

TABLE 1 Admit- Imped- tance ance T₁₀ T₉ T₈ T₇ T₆ T₅ T₄ T₃ T₂ T₁ T₀ (Y =1/Z) (Z) 0 0 0 0 0 0 0 0 0 0 1 1 1 0 0 0 0 0 0 0 0 0 1 1 1 + .1 .909 0 00 0 0 0 0 0 1 1 1 1 + .2 .833 0 0 0 0 0 0 0 1 1 1 1 1 + .3 .769 0 0 0 00 0 1 1 1 1 1 1 + .4 .714 0 0 0 0 0 1 1 1 1 1 1 1 + .5 .667 0 0 0 0 1 11 1 1 1 1 1 + .6 .625 0 0 0 1 1 1 1 1 1 1 1 1 + .7 .588 0 0 1 1 1 1 1 11 1 1 1 + .8 .555 0 1 1 1 1 1 1 1 1 1 1 1 + .9 .526

In the example thermometer code of TABLE 1, the controllable range ofthe output impedance is limited to between 1 and 0.526, and thesensitivity is 0.1 or 10% change in admittance for each step. As alsoillustrated by TABLE 1, a pure thermometer code requires one bit foreach step. Accordingly, one of the drawbacks of a pure thermometer codeis the large number of bits (and therefore control lines) required toallow a large range of output impedance. The number of control linesincreases exponentially as the degree of required step sensitivityincreases. For example, if it would be desirable to step the admittanceonly 1% in order to increase the sensitivity of each step, the PVTcontrol circuit would require 101 control lines, or tenfold the numberof lines required for adjusting it to the nearest 10%. Alternatively, ifit were desired to increase the range of adjustable output impedancefrom 1 to 0.25, in the example of TABLE 1 with each step changing theadmittance by 10%, an additional twenty bits (control lines) would berequired.

Accordingly, although an increased sensitivity range for adjusting thePVT output impedance is desirable, the number of bits required toimplement any significant range and/or sensitivity using a purethermometer code is outweighed by the added design complexity and chipreal estate required to implement it.

An alternative solution to use of a pure thermometer code is the use ofa pure binary weighted code whereby each leg of the PVT control circuitis binary weighted to comprise a resistive device having an admittancecorresponding to a combination of its binary weighted bit position. Inother words, each impedance leg in the impedance network has anadmittance of 2^((bit position))Y, where Y is a predefined minimumadmittance appropriate to the design. In other words, if bit B₀ of abinary-coded calibration word B_(0::n-1) controls a FET with admittanceY, bit B₁ of the calibration word B_(0::n-1) controls a FET withadmittance 2*Y, bit B₂ of the calibration word B_(0::n-1) controls a FETwith admittance 4*Y, and so on. Thus, the impedance of each leg of theimpedance network corresponds to the weighted position of the bit in thebinary code that controls the leg. More particularly, if the calibrationword B_(0::n-1) comprises 4 bits, the impedance leg controlled by bit B₀has a relative impedance weighting of 1. Similarly, the impedance legcontrolled by bit B₁ has a relative impedance weighting of 2, and theimpedance leg controlled by bit B₂ has a relative impedance weighting of4, and the impedance leg controlled by bit B₃ has a relative impedanceweighting of 8. In effect, as the binary count of the calibration wordB_(0::n-1) increments, more impedance is added in parallel in theimpedance network, and the output impedance on the signal pad drops.TABLE 2 illustrates an example of a pure 4-bit binary weighted code.

TABLE 2 Admittance Impedance B₃ B₂ B₁ B₀ (Y = 1/Z) (Z) 0 0 0 0 0infinite 0 0 0 1 0.1 10 0 0 1 0 0.2 5 0 0 1 1 0.3 3.333 0 1 0 0 0.4 2.50 1 0 1 0.5 2 0 1 1 0 0.6 1.667 0 1 1 1 0.7 1.429 1 0 0 0 0.8 1.25 1 0 01 0.9 1.11 1 0 1 0 1.0 1 1 0 1 1 1.1 .909 1 1 0 0 1.2 .833 1 1 0 1 1.3.769 1 1 1 0 1.4 .714 1 1 1 1 1.5 .667

As illustrated in TABLE 2, the benefit of using a binary weighted codeis its ability to achieve a larger range of output impedance using fewerbits (or PVT control lines). However, in a pure binary weighted code, astep-wise increment does not ensure that all legs currently activatedwill remain activated at the next step. This can result in a jump inoutput impedance, which may cause partial reflection of a signalincoming from a transmission line coupled to the output pad. Further, ifa transmission line is low because one of the drivers is active, theimpedance transition may launch a wave onto the transmission line. Thiscan result in an unacceptable noise glitch. For example, suppose thecurrent binary weighted calibration code B that controls a 4-legimpedance network has a value of binary 0111 (corresponding to anadmittance of 0.7) and the code B is to be incrementally stepped tobinary 1000 (corresponding to an admittance value of 0.8). When theelectrical connections of the impedance legs are switched from 0111 to1000, it is possible that for a very short time the switches may be in astate such that all the switched impedance legs are simultaneouslymomentarily on (corresponding to a binary value of “1111”) orsimultaneously momentarily off (corresponding to a binary value of“0000”). In this example and according to TABLE 2, the output impedanceZ_(OUT) could momentarily change from Z_(OUT)=1.429 (corresponding to anadmittance of 0.7) to Z_(OUT)=0.667 (corresponding to an admittance of1.5, or from Z_(OUT)=1.429 (corresponding to an admittance of 0.7) toZ_(OUT)=infinite (corresponding to an admittance of 1.5). As illustratedby this example, the more bits in the code that must be switched betweenone step value to the next increases the probability of an undesirablespike in the output impedance seen on the signal pad.

Hybrid PVT codes have been developed, for example as described in U.S.Pat. No. 6,326,802 to Newman et al., entitled “On-Die AdaptiveArrangements For Continuous Process, Voltage, And TemperatureCompensation”, herein incorporated by reference for all that it teaches.In the technique described therein, PVT compensation is achieved using ahybrid binary/linear (thermometer) adaptive arrangement duringinitialization to initially adapt to an external resistor R_(EXT), andthereafter locks the binary adaptive arrangement to the adaptedimpedance such that the binary adaptive arrangement cannot generateimpedance/noise glitches after initialization. Once the binary adaptivearrangement has been used to initially adapt to R_(EXT) duringinitialization then the hybrid binary/linear adaptive arrangementutilizes a linear (thermometer) adaptive arrangement to continuouslyreadapt to R_(EXT) to compensate for operational and environmentalvariations after initialization. However, while the hybrid binary/lineararrangement of U.S. Pat. No. 6,326,802 may be used to decrease thenumber of bits used for PVT compensation, the binary and lineararrangements are mutually exclusive during operation. Thus, duringinitialization, the output impedance is susceptible to impedance/noiseglitches, and after initialization, the sensitivity range is limited bythe number of bits lines.

Accordingly, a need exists for a PVT control encoding technique thatallows for a higher output impedance range with fewer control lines,while preventing spikes in the output impedance on the signal pads.

SUMMARY OF THE INVENTION

The present invention is a novel variable impedance output drivercontrol circuit and method for programming the same that employs ahybrid binary/thermometer coded adaptive scheme when adjusting theoutput impedance of an output driver to take into account variations inprocess, voltage, and temperature. The coding scheme of the inventionachieves a high range of sensitivity with a minimal number of controllines while preventing spikes in the output impedance on the signal pad.

In accordance with one embodiment of the invention, a hybridbinary/thermometer coded adaptive scheme is employed in a variableimpedance network to variably adjust the output impedance of an outputdriver circuit. The variable impedance network is configured with aplurality of impedance legs electrically connected between a voltagesource and a node that is electrically coupled to the output of theoutput driver circuit. Preferably, the sets of impedance legs stepped bythe binary stepped bits are weighted in increasing order of admittance,and in particular, in a binary weighting in order of the respectivepredetermined bit position of its corresponding calibration signal inthe calibration word.

Combinations of the impedance legs are connected and disconnected fromthe node according to a hybrid binary/thermometer code of the invention.In an exemplary embodiment, the impedance legs are partitioned into aplurality of sets of binary stepped impedance legs and correspondingthermometer stepped impedance legs. A calibration word having aplurality of bits each corresponding to and controlling a different oneof the impedance legs is partitioned into a plurality of sets of binarystepped bits and corresponding thermometer stepped bits. One pair of aset of binary stepped bits and corresponding set of thermometer steppedbits is activated for stepping at a time. Each set of binary steppedbits is stepped according to a binary code, and each corresponding setof thermometer stepped bits is stepped according to a thermometer codeonce per full count iteration of its corresponding set of binary bits.

On an increment step, if both the binary bits and correspondingthermometer bits of all active pairs of bit sets are incrementallysaturated (i.e., all bits in all active set pairs are “1”), a next pairof sets of binary stepped bits and corresponding thermometer steppedbits is activated. The cumulative set of activated binary stepped bitsis stepped according to a binary code, and the next set ofthermometer-stepped bits is stepped according to a thermometer code onceper full count iteration of the cumulative set of activated binary bits.

On a decrement step, if both the binary bits and correspondingthermometer bits of a given pair of bit sets are decrementally saturated(i.e., all bits in the set pair are “0”), the pair of sets of binarystepped bits and corresponding thermometer stepped bits are deactivated.The cumulative set of activated binary stepped bits is stepped accordingto a binary code, and the most recently activated set ofthermometer-stepped bits is stepped according to a thermometer code onceper full count iteration of the cumulative set of activated binary bits.

The process may be repeated as necessary to match the output impedanceof the output driver circuit to the load impedance of the output pad, oruntil all bits of the calibration word (and hence, all impedance legs)are activated.

BRIEF DESCRIPTION OF THE DRAWING

The invention will be better understood from a reading of the followingdetailed description taken in conjunction with the drawing in which likereference designators are used to designate like elements, and in which:

FIG. 1 is a schematic block diagram of a variable impedance outputdriver circuit;

FIG. 2 is a schematic diagram of an impedance network implemented inaccordance with the invention for the variable impedance output driverof FIG. 1;

FIG. 3A is a map of a flowchart of the method of the invention detailedin FIGS. 3B and 3C;

FIG. 3B is a first part of the flowchart of a preferred embodiment ofthe invention;

FIG. 3C is a second part of the flowchart of the preferred embodiment ofthe invention;

FIG. 4 is a schematic block diagram of a control circuit for variablyadjusting an output impedance of an impedance networks.

DETAILED DESCRIPTION

A novel method and system for increasing the range and sensitivity of avariable impedance output driver control circuit is described in detailhereinafter. Although the invention is described in terms of specificillustrative embodiments, it is to be understood that the illustrativeembodiments are shown by way of example only and the scope of theinvention is not intended to be limited thereby.

Turning now to FIG. 1, there is shown a block diagram illustrating avariable impedance output driver circuit 1. As shown, the driver circuit1 includes a pull-up predriver circuit 2 which operates to drive anoutput value from a low state to a high state (e.g., logic zero to logicone), and a pull-down predriver circuit 4 which operates to drive anoutput value from a high state to a low state. The pull-up predrivercircuit 2 controls a switching device 3, implemented in the illustrativeembodiment by an N-channel field effect transistor (NFET), thatswitchably connects a node 6 coupled to the output pad 8 to a firstvoltage source, e.g., DVDD. Similarly, the pull-down predriver circuit 4controls a switching device 5, also implemented in the illustrativeembodiment by an NFET, that switchably connects the node 6 coupled tothe output pad 8 via conductor 7 to a second voltage source, e.g., thechip ground. An impedance network 10 is interposed between the node 6and the driver pad 8. The impedance network 10 provides a controllablyvariable impedance, which serves to vary the output impedance of thedriver circuit 1 on signal pad 8 to match the board trace impedance.Control circuit 9 controls the impedance provided by the impedancenetwork 10. The output of the impedance network 10 is routed to thedriver pad 8 via conductor 7.

To more particularly describe the structure and operation of theimpedance network 10, reference is now made to FIG. 2, which illustratesthis circuitry in more detail. Specifically, the impedance network 10 isshown in relation to the node 6 and conductor 7, and control circuit 9.The impedance network 10 includes an impedance leg 20 that is connectedbetween node 6 and conductor 7, and a plurality of impedance legs 11–17programmably electrically connectable in parallel between the node 6 andconductor 7 by a control circuit 9. In the preferred embodiment, theimpedance legs 11–17 and 20 are implemented with field effecttransistors (FETs), but could be implemented using other components suchas a resistor in series with a switch. In the preferred embodiment, eachof the FETs 11–17 and 20 is defined by a channel width that defines theadmittance of that FET device. When activated (i.e., turned on toconduct current), each FET provides an electrical connection betweennode 6 and conductor 7 thereby allowing current flow and contributing tothe combined parallel resistance of the impedance network. When morethan one of the FET devices 11–17 is turned on, the characteristicresistance of the enabled FETs combine in parallel to provide a lowercombined resistance. In this way, the output impedance of the drivercircuit 1 may be varied.

The impedance leg 20 is always activated, allowing a signal to pass fromnode 6 to conductor 7, to prevent impedance jumps which can result innoise glitches on the output pad 8 that may occur momentarily as aresult of the switching on or off of the impedance legs 11–17.

The control circuit 9 generates a digital calibration word W_(1::7) toactivate selected ones of the switchable impedance legs 11–17 toprecisely control the output impedance of the driver circuit 1 inaccordance with the method described hereinafter in FIGS. 3A–3C. Eachrespective bit in the calibration word W_(1::7) corresponds to, andcontrols, a different one of the impedance legs 11–17. In the preferredembodiment, each respective bit W₁ through W₇ of the calibration wordW_(1::7) drives a different respective gate of a correspondingrespective FET device implementing the respective correspondingimpedance legs 11–17.

In the illustrative embodiment, the admittance of impedance legs 11–17of the impedance network 10 are weighted relative to the admittance ofimpedance leg 20 as follows: impedance leg 11 is characterized by anadmittance value of 10% of the admittance value of “always on” impedanceleg 20; impedance leg 12 is characterized by an admittance value that istwo times that of the first impedance leg 11; impedance leg 13 ischaracterized by an admittance value that is four times that of thefirst impedance leg 11; impedance leg 14 is characterized by anadmittance value that is four times that of the first impedance leg 11;impedance leg 15 is characterized by an admittance value that is fourtimes that of the first impedance leg 11; impedance leg 16 ischaracterized by an admittance value that is eight times that of thefirst impedance leg 11; and impedance leg 17 is characterized by anadmittance value that is eight times that of the first impedance leg 11.

The impedance legs 11–17 are partitioned into a plurality of sets 21,22,23, and 24. Impedance leg set 21 includes impedance legs 11 and 12;impedance leg set 22 includes impedance leg 13; impedance leg set 23includes impedance legs 14 and 15; and impedance leg set 24 includesimpedance legs 16 and 17.

TABLE 3 illustrates an exemplary hybrid binary/thermometer code inaccordance with the invention for a 7-bit output impedance calibrationword W_(1::7) for controlling the impedance network 10 of FIG. 2. Eachbit position W₀–W₇ in TABLE 3 corresponds to a control signal forcontrolling the activation (represented by a “1” in TABLE 3) ordeactivation (represented by a “0” in TABLE 3) of a respective impedanceleg 11–17. Thus, when the value of a bit in TABLE 3 is defined as a “0”,the impedance leg corresponding to the bit is electrically disconnectedfrom between node 6 and node 7, and therefore does not contribute to theparallel resistance of the impedance network 10. Likewise, when thevalue of a bit in TABLE 3 is defined as a “1”, the impedance legcorresponding to the bit is electrically connected between node 6 andnode 7, and therefore contributes to the parallel resistance of theimpedance network 10.

TABLE 3 Leg Bit Position Admittance Impedance Weighting W₇ W₆ W₅ W₄ W₃W₂ W₁ W₀ (Y = 1/Z) (Z) Count #0 0 0 0 0 0 0 0 1 1 1 Count #1 0 0 0 0 0 01 1 1 + .1 = 1.1 .909 Count #2 0 0 0 0 0 1 0 1 1 + .2 = 1.2 .833 Count#3 0 0 0 0 0 1 1 1 1 + .2 + .1 = 1.3 .769 Count #4 0 0 0 1 0 0 0 1 1 +.4 = 1.4 .714 Count #5 0 0 0 1 0 0 1 1 1 + .4 + .1 = 1.5 .667 Count #6 00 0 1 0 1 0 1 1 + .4 + .2 = 1.6 .625 Count #7 0 0 0 1 0 1 1 1 1 + .4 +.2 + .1 = 1.7 .588 Count #8 0 0 1 1 0 0 0 1 1 + .4 + .4 = 1.8 .555 Count#9 0 0 1 1 0 0 1 1 1 + .4 + .4 + .1 = 1.9 .526 Count #10 0 0 1 1 0 1 0 11 + .4 + .4 + .2 = 2.0 .500 Count #11 0 0 1 1 0 1 1 1 1 + .4 + .4 + .2 +.1 = 2.1 .476 Count #12 0 0 1 1 1 0 0 1 1 + .4 + .4 + .4 = 2.2 .455Count #13 0 0 1 1 1 0 1 1 1 + .4 + .4 + .4 + .1 = 2.3 .435 Count #14 0 01 1 1 1 0 1 1 + .4 + .4 + .4 + .2 = 2.4 .417 Count #15 0 0 1 1 1 1 1 11 + .4 + .4 + .4 + .2 + .1 = 2.5 .400 Count #16 0 1 1 1 0 0 0 1 1 + .8 +.4 + .4 = 2.6 .385 Count #17 0 1 1 1 0 0 1 1 1 + .8 + .4 + .4 + .1 = 2.7.370 Count #18 0 1 1 1 0 1 0 1 1 + .8 + .4 + .4 + .2 = 2.8 .357 Count#19 0 1 1 1 0 1 1 1 1 + .8 + .4 + .4 + .2 + .1 = 2.9 .345 Count #20 0 11 1 1 0 0 1 1 + .8 + .4 + .4 + .4 = 3.0 .333 Count #21 0 1 1 1 1 0 1 11 + .8 + .4 + .4 + .4 + .1 = 3.1 .323 Count #22 0 1 1 1 1 1 0 1 1 + .8 +.4 + .4 + .4 + .2 = 3.2 .313 Count #23 0 1 1 1 1 1 1 1 1 + .8 + .4 +.4 + .4 + .2 + .1 = 3.3 .303 Count #24 1 1 1 1 0 0 0 1 1 + .8 + .8 +.4 + .4 = 3.4 .294 Count #25 1 1 1 1 0 0 1 1 1 + .8 + .8 + .4 + .4 + .1= 3.5 .286 Count #26 1 1 1 1 0 1 0 1 1 + .8 + .8 + .4 + .4 + .2 = 3.6.278 Count #27 1 1 1 1 0 1 1 1 1 + .8 + .8 + .4 + .4 + .2 + .1 = 3.7.270 Count #28 1 1 1 1 1 0 0 1 1 + .8 + .8 + .4 + .4 + .4 = 3.8 .263Count #29 1 1 1 1 1 0 1 1 1 + .8 + .8 + .4 + .4 + .4 + .1 = 3.9 .256Count #30 1 1 1 1 1 1 0 1 1 + .8 + .8 + .4 + .4 + .4 + .2 = 4.0 .250Count #31 1 1 1 1 1 1 1 1 1 + .8 + .8 + .4 + .4 + .4 + .2 + .1 = 4.1.244

In accordance with the exemplary embodiment of the invention shown inFIG. 2, and as also illustrated in TABLE 3, bit W₀ which controlsactivation of impedance leg 20 is always activated (i.e., the gate ofFET 20 is tied to V_(DD) to allow saturating current flow throughimpedance leg 20 between node 6 and conductor 7). Impedance leg 20defines the highest impedance. Alternatively, because impedance leg 20is always activated, the switching device of the impedance leg 20 can behardwired to be always on, or implemented with a device that is notswitchable (e.g., a standard resistor) eliminating the need for thecontrol circuit 9 to generate bit W₀ in the calibration word.

As illustrated in TABLE 3, calibration word bits W₃, W₂, W₁ countaccording to a pseudo-binary counting scheme. As previously described,the impedance legs 11, 12, and 13 are respectively characterized byadmittances Y₁, Y₂, Y₃ of 10%, 20%, and 40% of Y₀, the admittance ofimpedance leg 20. Thus, if impedance leg FET 20 is weighted with anadmittance Y₀, impedance leg FET 11 is weighted with an admittanceY₁=10%*Y₀, impedance leg 12 is weighted with an admittance Y₂=2*Y₁, andimpedance leg FET 13 is weighted with an admittance Y₃=4*Y₁.

In contrast, calibration word bits W₇, W₆, W₅, W₄ count according to athermometer counting scheme. In the embodiment of FIG. 2, the impedancelegs 14, 15, 16, and 17 are preferably respectively characterized byadmittances Y₇, Y₆, Y₅, Y₄ of 80%, 80%, 40%, and 40% of Y₀. Accordingly,impedance leg FET 14 is weighted with an admittance Y₄=4*Y₁, impedanceleg FET 15 is weighted with an admittance Y₅=4*Y₁, impedance leg 16 isweighted with an admittance Y₂=8*Y₁, and impedance leg FET 17 isweighted with an admittance Y₃=8*Y₁.

The goal of the control circuit 9 is to switch the minimum number ofimpedance legs between count steps while maintaining a high degree ofsensitivity and impedance range. To accomplish this, the control circuit9 implements the hybrid binary/thermometer code of TABLE 3 when steppingthe calibration word W_(1::7) up or down.

When the impedance legs are weighted according to the illustrativeembodiment, the sensitivity of the impedance network when steppedaccording to the hybrid binary/thermometer code defined in TABLE 3 is10%. Thus, an increment or decrement of the programmed calibration wordW_(7::1) will result in a 10% change in admittance from the previousstep. In addition, the range of the impedance network 10 is 1 to 4.1,which, by comparison to TABLE 2, is much higher than a pure thermometercode can yield with the same number of bits. For example, if theimpedance of the impedance leg FET 20 is 10 ohms, then the minimumimpedance that the impedance network can yield is 10/4.1 or 24.39 ohms.A pure thermometer code having the same number of impedance legs wouldyield 10/1.7 or 58.8 ohms. Although use of a pure binary code having thesame number of impedance legs would yield a higher range, as discussedin the background section, a pure binary code implementation isproblematic because it exposes the output pad to a high prevalence ofimpedance spikes when switching between calibration word steps. Thus, itwill be appreciated that the hybrid binary/thermometer coding scheme ofthe invention affords the sensitivity of a pure binary coded scheme andincreased impedance range over a pure thermometer coding scheme.

FIG. 3A is a map of a flowchart of an exemplary embodiment of a method100 of the invention for stepping a hybrid binary/thermometer code. Themethod 100 is shown in two parts 100 a in FIGS. 3B and 100 b in 3C. Inthis method 100, it is assumed that the bits of the hybridbinary/thermometer code are partitioned into sets of binary bits andcorresponding sets of thermometer bits. When used to implement thecontrol circuit 9 in FIG. 2 to generate the calibration word W_(1::7),bits W₁ and W₂ form one binary bit set, and bits W₄ and W₅ form itscorresponding thermometer bit set. Bit W₃ forms another binary bit set,and bits W₆ and W₇ form its corresponding thermometer bit set.

Turning now in detail to the method 100, and in particular to methodpart 100 a in FIG. 3B, the method 100 begins with initialization ofvariables (step 101). In the illustrative preferred embodiment,initialization involves initialization of variables HBTCode (whichrepresents the hybrid binary/thermometer code generated by thecontroller, and includes all of the hybrid binary/thermometer code bitscontrolling the impedance legs of the impedance network), BinaryCode(which keeps track of the active binary bits in the HBTCode),ThermometerCode (which keeps track of the active thermometer bits in theHBTCode), IncrementallySaturated (which is a Boolean variable set toTrue only when the variable HBTCode is incrementally saturated (i.e.,all bits of the variable HBTCode are set to “1”)), andDecrementallySaturated (which is a Boolean variable set to True onlywhen the variable HBTCode is decrementally saturated (i.e., all bits ofthe variable HBTCode are set to “0”)). Initialization (step 101) resultsin setting all bits of each of HBTCode, BinaryCode, and ThermometerCodeto “0”, IncrementallySaturated to False, and DecrementallySaturated toTrue. All set pairs of binary bits and corresponding thermometer bitsare entered into a collection of available set pairs.

The method 100 then determines whether any available set pairs of binarybits and corresponding thermometer bits are available in the collectionof available set pairs (step 102). Immediately after initialization, thenumber of available set pairs will be maximized, and accordingly, themethod 100 selects a set of binary bits and its corresponding set ofthermometer bits from the collection of available set pairs (step 104),thereby removing the selected sets from the collection of available setpairs. The selection of bit pairs is preferably performed according to apriority scheme. In the illustrative embodiment, the priority schemeinvolves selection of the set comprising the lowest order binary bits inthe collection of set pairs, and its corresponding thermometer bits set.

The method 100 then activates the binary bits in the variable BinaryCodethat correspond to the binary bits in the selected set of binary bits(step 105). The bits in the variable BinaryCode correspond to like bitsin the variable HBTCode. If a bit in the variable BinaryCode is set to a“1”, then the corresponding bit in the HBTCode is active and is includedduring a binary increment or decrement of the binary bits of the HBTCode(discussed hereinafter). If a bit in the variable BinaryCode is set to a“0”, then the corresponding bit in the HBTCode is inactive and is notincluded during a binary increment or decrement of the binary bits ofthe HBTCode. Accordingly, a binary bit in the variable HBTCode isactivated by setting the corresponding binary bits in the variableBinaryCode to a “1”, and is deactivated by setting the correspondingthermometer bit in the variable BinaryCode to a “0”.

The method 100 then activates the thermometer bits in the variableThermometerCode that correspond to the thermometer bits in the selectedset of thermometer bits (step 106). The bits in the variableThermometerCode correspond to like bits in the variable HBTCode. If abit in the variable ThermometerCode is set to a “1”, then thecorresponding bit in the HBTCode is active and is included during athermometer increment or decrement of the thermometer bits of theHBTCode (discussed hereinafter). If a bit in the variableThermometerCode is set to a “0”, then the corresponding bit in theHBTCode is inactive and is not included during a thermometer incrementor decrement of the thermometer bits of the HBTCode. Accordingly, athermometer bit in the variable HBTCode is activated by setting thecorresponding thermometer bit in the variable ThermometerCode to a “1”,and is deactivated by setting the corresponding thermometer bit in thevariable ThermometerCode to a “0”.

The method 100 then waits for a step instruction (step 107). Uponreceipt of a step instruction, the method 100 determines whether thestep is to be in the “up” direction or the “down” direction (step 108).

If the step instruction indicates that the step is to be in the “up”direction, the method 100 determines whether the variable HBTCode isincrementally saturated (i.e., whether all bits in the variable HBTCodehave a value of “1”) (step 109). If so, the hybrid binary/thermometercode is already incremented to its limit, so the method 100 returns towait for a next step instruction (step 107). If the variable HBTCode isnot incrementally saturated, the method 100 performs a binary incrementof the active binary bits (as indicated by the variable BinaryCode) ofvariable HBTCode (step 110). The method 100 then determines whether thebinary increment resulted in an overflow of the active binary bits (step111). If not, the method 100 returns to wait for a next step instruction(step 107). If, however, the binary increment did result in an overflowof the active binary bits (as determined in step 111), the method thenperforms a thermometer increment of the active thermometer bits (asindicated by the variable ThermometerCode) of variable HBTCode (step112). The method 100 then determines whether the thermometer incrementresulted in saturation of the active thermometer bits (step 113). Ifnot, the method 100 returns to wait for a next step instruction (step107). If, however, the thermometer increment did result in saturation ofthe active thermometer bits (as determined in step 113), the method thendetermines whether there are any existing available set pairs of binarybits and thermometer bits in the collection of available set pairs (step102). If there are existing available set pairs of binary bits andthermometer bits in the collection of available set pairs, the methodselects the next set pair of binary bits and thermometer bits (step104), activates the binary bits in the HBTCode corresponding to thebinary bits in the selected set of binary bits (step 105), activates thethermometer bits in the HBTCode corresponding to the thermometer bits inthe selected set of thermometer bits (step 106), and returns to wait forthe next step instruction (step 107).

If there are no more existing available set pairs of binary bits andthermometer bits in the collection of available set pairs, the variableHBTCode is incrementally saturated, and the method 100 sets the variableIncrementallySaturated to True, and returns to wait for the next stepinstruction (step 107).

Returning now to receipt of a step instruction (step 107), if the stepinstruction indicates that the step is to be in the “down” direction (asdetermined in step 108), the method 100 enters method part 100 b, shownin FIG. 3C, where it determines whether the variable HBTCode isdecrementally saturated (i.e., whether all bits in the variable HBTCodehave a value of “0”) (step 114). If so, the hybrid binary/thermometercode is already decremented to its limit, so the method 100 returns towait for a next step instruction (step 107). If the variable HBTCode isnot decrementally saturated, the method 100 performs a binary decrementof the active binary bits (as indicated by the variable BinaryCode) ofvariable HBTCode (step 115). The method 100 then determines whether thebinary decrement resulted in an underflow of the active binary bits(step 116). If not, the method 100 returns to wait for a next stepinstruction (step 107). If, however, the binary decrement did result inan underflow of the active binary bits (as determined in step 116), themethod then performs a thermometer decrement of the active thermometerbits (as indicated by the variable ThermometerCode) of variable HBTCode(step 117). The method 100 then determines whether the thermometerdecrement resulted in decremental saturation of the active thermometerbits (i.e., all active thermometer bits are “0”) (step 118). If not, themethod 100 returns to wait for a next step instruction (step 107). If,however, the thermometer increment did result in decremental saturationof the active thermometer bits (as determined in step 113), the method100 then determines and deactivates the most recently activated set ofbinary bits in the variable BinaryCode, and adds the deactivated set ofbinary bits to the collection of available set pairs (step 119). Themethod 100 then determines and deactivates the most recently activatedset of thermometer bits in the variable ThermometerCode, and adds thedeactivated set of thermometer bits to the collection of available setpairs (step 120).

The method 100 then determines whether the variable HBTCode isdecrementally saturated (i.e., whether any remaining sets of activebinary bits and thermometer bits exist in the variables BinaryCode andThermometerCode) (step 121). If the variable HBTCode is notdecrementally saturated, the method 100 returns to wait for the nextstep instruction (step 107). If, however, the variable HBTCode isdecrementally saturated, and the method 100 sets the variableDecrementallySaturated to True (step 122), and returns to wait for thenext step instruction (step 107).

FIG. 4 is a schematic block diagram of an example control circuit 200that could be used to implement the method 100 of FIGS. 3A–3C and/orcontrol circuit 9 for programming the impedance network 10 of FIGS. 1and 2. As shown, the control circuit 200 includes a non-saturating 2-bitbinary up/down counter 202, a saturating 2-bit thermometer up/downcounter 204, a non-saturating 1-bit binary up/down counter 206, and asaturating 2-bit thermometer up/down counter 208.

Binary up/down counters 202 and 206 perform a binary increment ordecrement on the binary value at their respective outputs according tothe directional state of input DIR when the clock input, CLK, isstrobed. The binary output values of counters 202 and 206 respectivelycorrespond to calibration word bit sets W_(1:2) and W₃. When the countis set to the highest output (i.e., all “1”s) and the direction is setto increment, non-saturating binary up/down counters 202 and 206 operateto roll over from the highest output to the lowest output (i.e., all“0”s) upon a receipt of a next clock cycle (assuming the counter isenabled); likewise, when the count is set to the lowest output (i.e.,all “0”s) and the direction is set to decrement, non-saturating binaryup/down counters 202 and 206 operate to roll over from the lowest outputto the highest output (i.e., all “1”s) upon a receipt of a next clockcycle (assuming the counter is enabled).

Thermometer up/down counters 204 and 208 perform a thermometer incrementor decrement on the thermometer value at their respective outputsaccording to the directional state of input DIR when the clock input,CLK, is strobed. The thermometer output values of counters 204 and 208respectively correspond to calibration word bit sets W_(4:5) andW_(6:7). Saturating thermometer up/down counters 204 and 208 do not rollover from the highest output to the lowest or visa-versa.

Each counter 202, 204, 206, 208 is configured with a respective enableinput EN which, when asserted, allows the respective counter toincrement or decrement (depending on the state of the direction inputDIR) once for each strobe of the clock input signal CLK. In order toimplement the hybrid binary/thermometer code of TABLE 3, each of thehigher-order counters 204, 206, and 208 are only enabled during certaintimes of the programming process. In the embodiment of FIG. 4, counter202 is always enabled—therefore, assuming a positive-true logic scheme,the enable input EN of the counter 202 is tied to V_(DD).

Counter 204 is enabled only when both binary bits W₁ and W₂ are “1” andthe direction signal DIR is set to increment (i.e., is set to a logichigh in a positive-true logic scheme), or when all binary bits W₁, W₂,and W₃ and thermometer bits W₆ and W₇ are each “0” and the directionsignal DIR is set to decrement (i.e., is set to a logic low in apositive-true logic scheme). Accordingly, the control circuit 200includes an AND gate 230 (or equivalent logic) with inputs W₁, W₂, andDIR, a NOR gate 232 (or equivalent logic) with inputs W₁, W₂, W₃, W₆,W₇, and DIR, and an OR gate 234 (or equivalent logic) with the outputsof the AND gate 230 and NOR gate 234 as inputs. The output of the ORgate 234 is connected to the enable input EN of the saturatingthermometer code up/down counter 204.

Counter 206 is enabled only when binary bits W₁ and W₂ and thermometerbits W₄ and W₅ are each “1” and the direction signal DIR is set toincrement, or when binary bits W₁ and W₂ and thermometer bits W₆ and W₇are each “0” and the direction signal DIR is set to decrement.Accordingly, the control circuit 200 includes an AND gate 240 (orequivalent logic) with inputs W₁, W₂, W₄, W₅, and DIR, a NOR gate 242(or equivalent logic) with inputs W₁, W₂, W₆, W₇, and DIR, and an ORgate 244 (or equivalent logic) with the outputs of the AND gate 240 andNOR gate 244 as inputs. The output of the OR gate 244 is connected tothe enable input EN of the non-saturating binary code up/down counter206.

Counter 208 is enabled only when binary bits W₁, W₂, and W₃ andthermometer bits W₄ and W₅ are each “1” and the direction signal DIR isset to increment, or when binary bits W₁, W₂, and W₃ are each “0” andthe direction signal DIR is set to decrement. Accordingly, the controlcircuit 200 includes an AND gate 250 (or equivalent logic) with inputsW₁, W₂, W₃, W₄, W₅, and DIR, a NOR gate 252 (or equivalent logic) withinputs W₁, W₂, and W₃ and DIR, and an OR gate 254 (or equivalent logic)with the outputs of the AND gate 250 and NOR gate 254 as inputs. Theoutput of the OR gate 254 is connected to the enable input EN of thesaturating thermometer code up/down counter 208.

The direction input DIR is controlled by a comparator circuit 210.Conductor 7 at the output of the impedance network 10 is an input to theinverting terminal of analog comparator 212. The non-inverting input ofanalog comparator 212 is connected to a voltage divider formed withresistive devices 214 and 216. Resistive devices 214 and 216 may beon-chip resistors (or equivalents thereof) and are connected in seriesbetween the positive supply and the negative supply with theintermediate node connected to the non-inverting input of analogcomparator 212. In one embodiment, resistors 214 and 216 have the samevalue so that the voltage at the non-inverting input of analogcomparator 212 is VDD/2. The output 222 of analog comparator 212 isconnected to the DIR input of each of counters 202, 204, 206, and 208,which controls the direction that the counters count, if enabled.

When the inverting input of comparator 212 is lower than thenon-inverting input of analog comparator 212, counters 202, 204, 206,and 208, count up (when enabled) to turn on more of the transistors ofimpedance network 10 to decrease the impedance of impedance network 10.This feedback system stabilizes when the impedance of impedance network10 nearly matches the impedance of the load driven by the pad 8. Whenthe inverting input of comparator 212 is higher than the non-invertinginput of analog comparator 212, the output 222 corresponding todirection signal DIR is 0, which instructs the counters 202, 204, 206,and 208 to count in the down direction. This turns off more of thetransistors of impedance network 10, thereby increasing the impedance ofimpedance network 10.

It will be appreciated that the method 100 of FIG. 3 and the controlcircuit 200 of FIG. 4 allow continuous and/or periodic adjustment to theoutput impedance of the output pad 8 of output driver circuit 1 viacontinuous and/or periodic adjustment of the calibration control wordW_(1::7).

The method 100 of FIGS. 3A–3C and the control circuit 200 of FIG. 4 areprovided herein merely as illustrative of how the calibration wordW_(1::7) that implements the hybrid binary/thermometer code of TABLE 3and controls the impedance network 10 may be generated. However, it willbe appreciated that various methods and circuits may be designed toimplement the hybrid binary/thermometer code of the invention that isused to vary the output impedance of the driver circuits so that theoutput resistance substantially matches the load impedance over variousranges of the process, voltage, and temperature. Consistent with theconcepts and teachings of the present invention, the control circuit 9(FIGS. 1 and 2) and/or method of generation of the hybridbinary/thermometer code of the invention (FIGS. 3A–3C) may beimplemented in a variety of ways, and the illustrative embodimentspresented herein are for purposes of illustration only and notlimitation.

The foregoing description has been presented for purposes ofillustration and description. It is not intended to be exhaustive or tolimit the invention to the precise forms disclosed. Obviousmodifications or variations are possible in light of the aboveteachings. For example, it will be appreciated that the number ofimpedance legs in the impedance network and corresponding number of bitsof the calibration word may be increased or decreased according to theneeds of the application.

It will also be appreciated that the weighting of the impedance legs,the number and allocation of impedance legs to impedance leg sets, andcorresponding calibration word bits to various pairs of sets of binarybits and thermometer bits may be varied to achieve the desired range andsensitivity of the impedance network.

It will also be appreciated that the variable impedance network may beconnected to more than one output pad and used to simultaneously andglobally adjust the output impedance of these more than one output pads.Further, although the illustrative embodiment illustrates the use of avariable impedance network in accordance with the invention for both thepull-up and the pull-down portions of a driver circuit, more preciseimpedance matching over various process, voltage, and temperature rangesmay be achieved by implementing separate impedance networks for each ofthe pull-up and the pull-down portions of a driver circuit, and may ormay not implement separate and independent calibration words for theseparate impedance networks of the pull-up and the pull-down portions ofa driver circuit.

Although the invention has been described in terms of the illustrativeembodiments, it will be appreciated by those skilled in the art thatvarious changes and modifications may be made to the illustrativeembodiments without departing from the spirit or scope of the invention.It is intended that the scope of the invention not be limited in any wayto the illustrative embodiment shown and described but that theinvention be limited only by the claims appended hereto.

1. A variable impedance network for variably adjusting the outputimpedance of a node of a circuit, comprising: one or more impedance legselectrically connected between a voltage source and said node; a firstset of switchable impedance legs programmably electrically connectablebetween said voltage source and said node by a first set of calibrationsignals each of which corresponds to a respective one or more of saidswitchable impedance legs in said first set of switchable impedancelegs; a second set of switchable impedance legs programmablyelectrically connectable between said voltage source and said node by asecond set of calibration signals each of which corresponds to arespective one or more of said switchable impedance legs in said secondset of switchable impedance legs; and a controller which generates saidfirst set of calibration signals and said second set of calibrationsignals, wherein said controller steps said first set of calibrationsignals according to a binary code and steps said second set ofcalibration signals according to a thermometer code once per full countiteration of said first set of calibration signals.
 2. A variableimpedance network in accordance with claim 1, wherein: said first set ofcalibration signals and said second set of calibration signals representbits with respective predetermined bit positions in a calibration word,and wherein each of said first set of switchable impedance legs isweighted increasingly in order of the respective predetermined bitposition of its corresponding calibration signal in the calibrationword.
 3. A variable impedance network in accordance with claim 2,wherein: each of said first set of switchable impedance legs is binaryweighted according to the bit position of its corresponding calibrationsignal in the calibration word.
 4. A variable impedance network inaccordance with claim 1, wherein: each of said switchable impedance legsin said first set of impedance legs and said second set of impedancelegs comprises a field effect transistor (FETs) sized to have acorresponding predetermined respective impedance.
 5. A variableimpedance network in accordance with claim 1, further comprising: athird set of switchable impedance legs programmably electricallyconnectable between said voltage source and said node by a third set ofcalibration signals each of which corresponds to a respective one ormore of said switchable impedance legs in said third set of switchableimpedance legs; a fourth set of switchable impedance legs programmablyelectrically connectable between said voltage source and said node by afourth set of calibration signals each of which corresponds to arespective one or more of said switchable impedance legs in said fourthset of switchable impedance legs; and wherein said controller generatessaid third set of calibration signals and said fourth set of calibrationsignals, wherein said controller steps said first set of calibrationsignals and said third set of calibration signals, taken as acombination, according to a binary code and steps said fourth set ofcalibration signals according to a thermometer code once per full countiteration of said combination of said first set of calibration signalsend said third set of calibration signals.
 6. A variable impedancenetwork in accordance with claim 5, wherein: said first set ofcalibration signals, said second set of calibration signals, said thirdset of calibration signals, and said fourth set of calibration signalsrepresent bits with respective predetermined bit positions in acalibration word, and wherein each of said first set of switchableimpedance legs and each of said third set of switchable impedance legsis weighted increasingly in order of the bit position of itscorresponding calibration signal in the calibration word.
 7. A variableimpedance network in accordance with claim 5, wherein: each of saidfirst set of switchable impedance legs and each of said third set ofswitchable impedance legs is binary weighted according to the bitposition of its corresponding calibration signal in the calibrationword.
 8. A variable impedance network in accordance with claim 5,wherein: each of said switchable impedance legs in said first set ofimpedance legs, said second set of impedance legs, said third set ofimpedance legs, and said fourth set of impedance legs comprises a fieldeffect transistor (FETs) sized to have a corresponding predeterminedrespective impedance.
 9. A method for variably adjusting the outputimpedance of a node of a circuit, said circuit comprising an impedancenetwork interposed between a voltage source and said node, saidimpedance network comprising one or more impedance legs electricallyconnected between said voltage source and said node, a first set ofswitchable impedance legs programmably electrically connectable betweensaid voltage source and said node by a first set of calibration signalseach of which corresponds to a respective one or more of said switchableimpedance legs in said first set of switchable impedance legs, and asecond set of switchable impedance legs programmably electricallyconnectable between said voltage source and said node by a second set ofcalibration signals each of which corresponds to a respective one ormore of said switchable impedance legs in said second set of switchableimpedance legs, said method comprising the steps of: stepping said firstset of calibration signals according to a binary code; and stepping saidsecond set of calibration signals according to a thermometer code onceper full count iteration of said first set of calibration signals.
 10. Amethod in accordance with claim 9, wherein said first set of calibrationsignals and said second set of calibration signals represent bits withrespective predetermined bit positions in a calibration word, saidmethod further comprising: weighting each of said first set ofswitchable impedance legs increasingly in order of the respectivepredetermined bit position of its corresponding calibration signal inthe calibration word.
 11. A method in accordance with claim 9, whereinsaid first set of calibration signals and said second set of calibrationsignals represent bits with respective predetermined bit positions in acalibration word, said method further comprising: weighting each of saidfirst set of switchable impedance legs according to a binary weightingscheme in order of the bit position of its corresponding calibrationsignal in the calibration word.
 12. A method in accordance with claim 9,wherein said impedance network further comprises a third set ofswitchable impedance legs programmably electrically connectable betweensaid voltage source and said node by a third set of calibration signalseach of which corresponds to a respective one or more of said switchableimpedance legs in said third set of switchable impedance legs, and afourth set of switchable impedance legs programmably electricallyconnectable between said voltage source and said node by a fourth set ofcalibration signals each of which corresponds to a respective one ormore of said switchable impedance legs in said fourth set of switchableimpedance legs, said method further comprising the steps of: steppingsaid first set of calibration signals and said third set of calibrationsignals, taken as a combination, according to a binary code; andstepping said fourth set of calibration signals according to athermometer code once per full count iteration of said combination ofsaid first set of calibration signals and said third set of calibrationsignals.
 13. A method in accordance with claim 12, said method furthercomprising: weighting each of said first set of switchable impedancelegs and said third set of impedance legs increasingly in order of therespective predetermined bit position of its corresponding calibrationsignal in the calibration word.
 14. A method in accordance with claim13, said method further comprising weighting each of said first set ofswitchable impedance legs according to a binary weighting scheme inorder of the bit position of its corresponding calibration signal in thecalibration word.
 15. A variable impedance network for variablyadjusting the output impedance of a node of a circuit, comprising: oneor more impedance legs electrically connected between a voltage sourceand said node; a first set of switchable impedance legs programmablyelectrically connectable between said voltage source and said node by afirst set of calibration signals each of which corresponds to arespective one or more of said switchable impedance legs in said firstset of switchable impedance legs; a second set of switchable impedancelegs programmably electrically connectable between said voltage sourceand said node by a second set of calibration signals each of whichcorresponds to a respective one or more of said switchable impedancelegs in said second set of switchable impedance legs; a third set ofswitchable impedance legs programmably electrically connectable betweensaid voltage source and said node by a third set of calibration signalseach of which corresponds to a respective one or more of said switchableimpedance legs in said third set of switchable impedance legs; a fourthset of switchable impedance legs programmably electrically connectablebetween said voltage source and said node by a fourth set of calibrationsignals each of which corresponds to a respective one or more of saidswitchable impedance legs in said fourth set of switchable impedancelegs; wherein said first set of calibration signals, said second set ofcalibration signals, said third set of calibration signals, and saidfourth set of calibration signals represent bits with differentrespective predetermined bit positions in a calibration word, andwherein each of said first set of switchable impedance legs and each ofsaid third set of switchable impedance legs is binary weighted accordingto the bit position of its corresponding calibration signal in thecalibration word; and a controller which generates said first set ofcalibration signals, said second set of calibration signals, said thirdset of calibration signals, and said fourth set of calibration signals;wherein said controller steps said first set of calibration signalsaccording to a binary code and steps said second set of calibrationsignals according to a thermometer code once per full count iteration ofsaid first set of calibration signals, and steps said first set ofcalibration signals and said third set of calibration signals, taken asa combination, according to a binary code and steps said fourth set ofcalibration signals according to a thermometer code once per full countiteration of said combination of said first set of calibration signalsand said third set of calibration signals.